charging question

OK my 98 300 tdi 90 has a charging issue.
I fitted a denso 65 amp alternator about 2 years ago,
It is still putting out about 14v on tick over, but once i start putting the lights ,heater etc on the voltage is dropping to 13.5 volts even with slight rev to the engine, which i know is a bit low. Sometimes on tick over it is dropping to 13.2 volts, but when i am actually driving it is steading around the 13.6 - 13.5 volts.

I am wondering if it has anything to do with the second battery that i have fitted with a x eng split charge.

Yes i know the sensible thing would be to disconnect the second battery and see if it makes a difference, this is not possible as i am working away just now and wont be home for another week or so. So if i do need a higher out put alternator i could order one and fit it when i get home.


I have 2 numax batteries 18 months and 2 years old.
The wiring from the alternator has already been beefed up as in the sumer i had 1.2 volt drop from the alternator to the starter.

Thoughts please

Doesn't sound abnormal to me - could be wrong

Mine drops below 13v once lights and heater blower are on

65 amp is at the lower end of alternators. But, unless you're having specific issues (or you're rich) I wouldn't rush at it, but you could easily double that output.
If it's possible I'd try a trickle charger first. No alternator fully charges a battery and a little assistance from a decent charger might help, especially if you're parking up for a week or so like now.

Station House said:

Doesn't sound abnormal to me - could be wrong

Mine drops below 13v once lights and heater blower are on

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Me too, Mine will charge at about 12.5V at idle with everything on, its not really charging but its supplying what im using, once im driving it goes up to 13.2-13.5V under full load. I am also charging a second battery mind and thats usually at about 13.5V unless I put the heated seats or winch on then it drops...

mine runs at between 14 & 15.2 volts on the first battery, 13.5 at tickover, stable 28.2 across the two, load doesn't show much change even when running all lights and eberspacher blow heater, however i am running a 2400 amp alternator on the tdi lol so wouldnt expect much drop

turn all load off and if battery and alternator are tip top u should see 14.5 volts with second battery isolated

sorry not much help i know


cheers steve

Generally, 13-15v is good.. outside of that is bad.

personally if it was a 'no load' situation i'd be unhappy to see anywhere near 13 volts, thats barely a trickle in charging terms..... any lucas alt should easily fire 14+ above tickover

cheers steve

I had my 300tdi running at 14.1V with no load last week. When you had lights and heater on it would drop to just under 13V.

Spent a few minutes with the battery dis-connected (in true Haynes style) and removed, cleaned and replaced all the heavy cables on the alternator and starter motor. We now have 13.8V on full load, which is good for keeping the battery topped up.
The battery is one of those nice heavy duty 663 ones that gives loads of cranking amps so needs a good charge

wermy said:

I had my 300tdi running at 14.1V with no load last week. When you had lights and heater on it would drop to just under 13V.

Spent a few minutes with the battery dis-connected (in true Haynes style) and removed, cleaned and replaced all the heavy cables on the alternator and starter motor. We now have 13.8V on full load, which is good for keeping the battery topped up.
The battery is one of those nice heavy duty 663 ones that gives loads of cranking amps so needs a good charge

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that sounds about right... full load 13.8 is good assume you'll now be seeing 14.5 ish no load, folks often neglect all the cables n connections and assume 13 volts to a 12 volt battery must be okay... in my experience thats a problem

certainly aa n rac will be looking for upwards of 14.. if not will tell u u have an issue

cheers steve

all the connections are clean and tight, It is the heavy duty numax batteries i have. An auto spark told me that i need at least 13.6 volts with it under load to make sure the batteries are charging, i Have tried the trickle charger.

Unfortunately my landy can lie for a few days un used and theres not much life form the starter, the starter is only a couple of years old as well and its a Bosch one.

Seem to have problems most winters as it seems as if the battery goes flat after a week, but i have checked for a parasitic drain and all is ok, i have swapped the batteries around, but even after a week or two with out use the winch will still go at its normal pace but the starter just seems as if the battery is flat(diff batteries). It seems to me that there isn't enough juice going into the starter battery to run everything and fully charge it.

Trying to rule things out before i go and spend the best part of £200 on a 100 amp denso alternator

andy mcd said:

all the connections are clean and tight, It is the heavy duty numax batteries i have. An auto spark told me that i need at least 13.6 volts with it under load to make sure the batteries are charging, i Have tried the trickle charger.

Unfortunately my landy can lie for a few days un used and theres not much life form the starter, the starter is only a couple of years old as well and its a Bosch one.

Seem to have problems most winters as it seems as if the battery goes flat after a week, but i have checked for a parasitic drain and all is ok, i have swapped the batteries around, but even after a week or two with out use the winch will still go at its normal pace but the starter just seems as if the battery is flat(diff batteries). It seems to me that there isn't enough juice going into the starter battery to run everything and fully charge it.

Trying to rule things out before i go and spend the best part of £200 on a 100 amp denso alternator

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if it sits for a time have u tried a battery isolator?? i know u haven't found a drain but for the sake of 2 or 3 quid i'd try one, added bonus that once the screw is removed any toerag trys to nick they will blow the fuse n no chance of starting.

cheers steve

Can't you measure it over a week with a multimeter?

just reread...winch runs okay but starter struggles??? that points to an earth issue on the starter / engine, have u tried an extra earth or earthing the starter with a jump lead to see if it improves?????

cheers steve

The earth cables were replaced in the summer with marine tinned cable, added a few extra, Cant fit a cut of switch as i have a cat 1 alarm and immobilizer fitted and if its not armed and working my insurance is invalid.

I have even tried a jump lead between the engine and the earth on the battery to see if it makes any difference, but it doesn't. Its as if the starter battery isn't being charged properly, Maybe the extra juice the second battery is drawing to charge is causing the starter battery to not charge properly...

The parasitic drain is less than what my car is, so it isnt to blame



Maybe i will just have to cough up for another alternator

andy mcd said:

The earth cables were replaced in the summer with marine tinned cable, added a few extra, Cant fit a cut of switch as i have a cat 1 alarm and immobilizer fitted and if its not armed and working my insurance is invalid.

I have even tried a jump lead between the engine and the earth on the battery to see if it makes any difference, but it doesn't. Its as if the starter battery isn't being charged properly, Maybe the extra juice the second battery is drawing to charge is causing the starter battery to not charge properly...

The parasitic drain is less than what my car is, so it isnt to blame



Maybe i will just have to cough up for another alternator

Click to expand...

the isolator i was thinking of still has a fused bypass to run an alarm when u remove the screw... but sounds as though u have covered all the bases

i would isolate the second battery via a relay or better still manually and only connect when on a long run and isolate 5 mins before end of journey........ would at least eliminate 2nd battery scenario.... or is that already the case

cheers steve

cheers steve

i am going to disconnect the second battery and run it for a few days to see if theres any difference. If not maybe my alternator doesn't like all the muddy water and rivers it has been submerged in

But i have to get 13.6-14.4v @ 3000 rpm according to the workshop manual

just a quick update,

i unplugged the split charge to isolate the second battery and the voltage increased by 1.3 volts.
better but still not good enough.

So tonight i fitted a brand new 100 amp denso alternator, what a difference.
with the heater on full, head lights on , rear screen heater on, fog light on and hazards lights on... i am getting 13.99 volts @ the battery on tick over.

The head lights look a lot brighter aswell.. so hopefully the end of my problems if not i will have to replace the starter with a recon heavy duty one

If you are charging 2 batteries you ought to up the alternator to a 100 amp one. 2 100amp batteries will take a while to charge up fully on a 65amp alternator.

Also something else to check is that both batteries are of different age, 18 months and 2 yrs, if one battery is bad or on its way out it will drain the other one leading to flat batterys if your running two.

two batteries are still very healthy, both numax ones.


what a difference driving the landy now, even the dash lights are brighter